Photo-ionization detectors (PIDs) can detect and measure the concentrations of volatile gases in a gas sample by ionizing the volatile gases and measuring a resulting ionization current. FIG. 1 illustrates a conventional PID 100 including an ultraviolet (UV) lamp 110, which typically produces high-energy photons having energy above about 8.4 electron volts (eV). The high-energy photons from UV lamp 110 are directed into an ionization chamber 120 through an optical window 116, which may be an integral part of UV lamp 110. When a UV photon collides with a volatile gas molecule having an ionization potential below the energy of the UV photon, the collision ionizes the volatile gas molecule freeing an electron and creating a detectable ion.
An ion detector 130 in PID 100 has a pair of electrodes 132 and 134 that are typically made of a metal. A high voltage (e.g., greater than 150 V) applied between electrodes 132 and 134 generates an electrical field that attracts positively-charged particles (e.g., ions) to electrode 132 and attracts negatively-charged particles (e.g., electrons) to electrode 134. Electrode 134 repels ions towards electrode 132 that is simultaneously collecting the volatile gas ions. As a result, a measurement current produced at electrode 132 indicates the number of ions collected. The magnitude of the measurement current (or a measurement signal generated from the measurement current) therefore depends on the concentration of ionizable gas molecules, the intensity of the UV light in ionization chamber 120, and the efficiency of ion detector 130. If the detector efficiency and the UV light intensity are constant, a one-to-one mapping can be used to convert the measurement signal to a concentration, e.g., in parts per million (ppm) of the volatile/organic compounds.
A conventional PID 100 has a drive circuit 112 that drives UV lamp 110 with a constant amplitude driver signal. However, with a constant driver signal, a variety of factors, including degradation of UV lamp 10, contamination of optical window 116, and the presence of interfering substances such as methane in ionization chamber 120 typically diminish the UV light intensity of UV lamp 110 during normal operation of PID 100.
U.S. Pat. No. 6,225,633 issued to Hong T. Sun and Peter C. Hsi describes a process for self-cleaning a PID and particularly the optical window in the ionization chamber. The self-cleaning process traps air or another gas containing oxygen in the ionization chamber and transmits UV light into the ionization chamber to create ozone. Circulation through the PID is restricted so that the ozone accumulates in the ionization chamber, and ozone, being a strong oxidant, etches and removes the contamination from the optical window and other surfaces in the ionization chamber. Although the cleaning of the window eliminates or reduces one cause of UV intensity reductions, the degradation of the UV lamp still causes the gradual loss of UV light intensity. As a result, the measurement current that a conventional PID detects for a specific gas concentration decreases over time.
FIG. 2 illustrates plots 210, 220, 230, 240, and 250 of the typical dependence of the measurement signal on the volatile gas concentration for several different UV intensities. A calibration operation for a conventional PI) feeds a span gas having a known concentration of volatile gases into the PID, activates the UV lamp, and selects the mapping 210, 220, 230, 240, or 250 that maps the resulting measurement signal to the known concentration of the span gas. The mapping selected during calibration is then used for measurements during normal operation. However, as a PID ages, the UV intensity from the lamp drops, and the selected mapping of the measurement signal to the gas concentration becomes inaccurate. To prevent drift in the concentration measurements, a conventional PID thus requires frequent calibration to reselect the correct mapping for the conversion of measurement signals to volatile gas concentrations.